Superconductive materials have the unique material property of having zero electrical resistance. In other words, superconductive materials can conduct electricity with no loss of energy. However, superconductive materials only exhibit this unique material property when cooled below their respective critical temperature. The critical temperature for superconductive materials is in the supercold or cryogenic range of temperatures.
Superconductive materials are particularly useful in applications that utilize a magnetic field. Such applications include, for example, electric motors and generators, transformers, magnetic energy storage devices, magnetic bearings, colliders, and the like. The magnetic field that can be generated using superconductive materials is far greater than the magnetic field that can be produced using conventional conductive materials, such as copper. For example, some applications of magnetic resonance imaging (MRI) require a high magnetic field that can only be generated using superconductive materials. The medical field applications of magnetic resonance imaging has saved countless lives.
In applications using superconductive materials, the magnetic field is generally produced by a superconducting coil that contains the superconductive material. The superconductive material is often in the form of a superconducting wire that is wrapped around a core. The superconductive wire produces a magnetic field inside and outside of the core. The magnetic field can be increased by increasing the number of times the superconductive wire is wrapped around the core and by increasing the current flowing through the superconductive wire. As will be discussed in greater detail below, the magnetic field produces a physical load on each individual superconducting wire. This physical load is generally referred to by those skilled in the art as Lorentz stresses.
Lorentz stresses are produced by the magnetic field acting on the superconductive materials and increase by the square of magnetic field strength. Lorentz stresses produce a mechanical, or operational, load that acts to push the individual superconducting wires away from the core. In conventional superconducting coils, the operational load is transferred outward from each superconducting wire to each outwardly successive superconducting wire until the entire operational load from all the inwardly preceding superconducting wires is transferred to an outer support structure that surrounds the superconducting wires. This is analogous to a stack of bricks, where the top brick only supports its own weight but the bottom brick must support the weight of the entire stack of bricks.
In very high magnetic field applications, the load supported by the outer superconducting wire can be greater than the physical strength of the semiconductor wire. The outer superconducting wire is essentially crushed by the operational loads from the inner superconducting wires. Accordingly, the strength of the magnetic field and the number of superconducting wires that can be layered in a conventional superconducting coil is limited.
As the current through the superconducting wires is increased to produce a very high magnetic field, the high operational loads on the superconducting wires can cause the shape of a conventional superconducting coil to change. The change in shape of the superconducting coil distorts the magnetic field produced by the superconducting coil.